The “Advanced Television Systems Committee-Mobile/Handheld” standard, commonly designated with the aid of the initials ATSC-MH, and referenced under the number ATSC A/153, is an upward compatible extension of the ATSC digital terrestrial television standard. It makes it possible to transport data while taking account of constraints specific to wireless transmissions. An ATSC-MH stream comprises a series of frames termed M/H frames, able to transport jointly, on the one hand so-called “legacy” data, that is to say data intended for conventional terrestrial receivers, and on the other hand data intended for mobile terminals.
As illustrated in FIG. 1, an M/H frame 101 is spread over a duration of 968 ms and is composed of 5 sub-frames 102. Each sub-frame is split into 16 temporal notches 103 called “slots”, a slot comprising 156 TS (“Transport Stream”) packets of 188 bytes each. A slot may be assigned to the transport of legacy data exclusively and therefore comprise 156 TS packets of legacy data, or else assigned to the transport of 118 packets dedicated to mobiles, the other 38 packets being reserved for legacy data. Furthermore, the standard prescribes that the allocation of the slots in each sub-frame of one and the same frame be identical. Consequently, when a sub-frame slot is allocated to a service, there are 5 slots of the MH frame which are allocated to this service (one slot for each of the 5 sub-frames). For the sake of simplification, the expression “allocation of a slot to a service” is understood to mean the fact that this slot is allocated to the transport of data related to this service.
FIG. 2 presents an example according to the prior art of allocating slots in an M/H frame. For the sake of clarity, the allocation of the slots to the various services is represented by a single sub-frame, the other four sub-frames, not represented, of the M/H frame being assigned an identical allocation of slots. The 16 slots of a sub-frame are numbered from 0 to 15. In the example of FIG. 2, two services—also dubbed “parade” in the ATSC-specific terminology—each occupy two slots, the first service A occupying slots 0 and 4, the second service B occupying slots 8 and 12. The order of allocation of the slots in the sub-frames which has been defined by the standard is as follows: slot 0, slot 4, slot 8, slot 12, slot 2, slot 6, slot 10, slot 14, slot 1, slot 5, slot 9, slot 13, slot 3, slot 7, slot 11, and slot 15.
Conventionally, an entity for formatting the ATSC-MH stream performs an allocation of slots on a pattern consisting of a frame. Stated otherwise, it chooses an allocation scheme which repeats from one frame to another, so that if the bitrate allocated to the first service A must be increased, the entity allocates an additional slot per sub-frame, thereby corresponding to a bitrate increase of the order of 1 Mbit/s (five new slots allocated per M/H frame, at a rate of 118 packets of 188 bytes per slot). This increase is relatively heavy and may be excessive in regard to the actual bitrate needs. By way of illustration, if the frames of a stream are formatted according to the scheme illustrated in FIG. 2, then the input bitrate associated with each of the services A and B is of the order of 2 Mbits/s. If the bitrate associated with the first service A must be heightened to 2.1 Mbits/s to satisfy the quality required by this service, then an additional slot per sub-frame must be allocated, thereby hoisting the bitrate to 3 Mbits/s for service A. As a result, the 900 Kbits/s allocated over and above the 2.1 Mbits/s are unused.
An aim of the invention is to prevent losses of useful bitrate from occurring in a stream of the ATSC-MH type because of the formatting of the M/H frames. For this purpose, the subject of the invention is a method of formatting a stream of data for mobile terminals according to a series of frames which comply with the ATSC-MH standard or with a standard which is upward compatible with ATSC-MH, a frame comprising a predetermined number of slots, each slot being able to be allocated to the transport of data associated with a service, the method comprising a step of allocating the slots to the various services to be transported, characterized in that it performs said allocating step on a set grouping together M frames, M being greater than 1, so that at least two frames from among the M frames comprise a different number of slots associated with one and the same service.
The method according to the invention makes it possible to adapt, in a stream of the ATSC-MH type, the bitrate allocated to the data intended for mobile terminals in a finer manner, and thus to avoid the overly large jumps in bitrate responsible for the losses experienced in the prior art.